The present invention generally relates to a plasma processing apparatus and a plasma processing method for effecting surface treatment such as dry etching or formation of a thin film by means of CVD (Chemical Vapor Deposition) by utilizing microwave plasma in the production of semiconductor devices, and more particularly to a plasma processing apparatus and a plasma processing method for effecting surface treatment by applying RF bias voltage to a substrate to be processed.
In a plasma processing apparatus for effecting etching or thin-film formation on a substrate in order to produce semiconductor devices, it is required to arbitrarily control, at the time of etching, the anisotropy of etching, damage to the substrate surface, and the processing speed of etching, and, at the time of thin-film formation, the composition of the film such as the state of coupling between atoms, film quality such as water permeability, stress imparted to the film, and step coverage, respectively. However, it is not easy to realize an apparatus having performance that simultaneously satisfies these various conditions. In recent years, a microwave plasma processing apparatus which employs ECR (Electron Cycrotron Resonance) plasma has attracted much attention as an apparatus having the possibility of satisfying these conditions. The ECR plasma is based on such a principle that the electrons are accelerated by making use of resonance effects created between a magnetic field and microwaves, and a gas is ionized by using kinetic energy of the electrons, thereby generating the plasma. The electrons excited by the microwaves make circular motions around the lines of magnetic force. In this case, a condition under which the centrifugal force equilibrates with the Lorentz force is referred to as an ECR condition. Let the centrifugal force be mr.omega..sup.2 and the Lorentz force be -qr.omega.B, then the condition of their equilibrium is expressed as .omega./B=q/m, where .omega. is the angular frequency of the microwave, B is the density of magnetic flux and q/m is the specific charge. A commonly used frequency of microwave is 2.45 GHz which is industrially certified. In this case, the resonance magnetic flux density is 875 gausses.
The ECR plasma etching/CVD apparatus requires a step of transforming the microwave into pulses having a large peak power preparatory to the impression thereof, which microwaves are introduced into the plasma generating chamber to generate the plasma, for the purpose of effecting highly efficient etching or forming the thin film by providing a high density of plasma. Besides, it is a common practice that the RF bias voltage is applied between the plasma and the substrate to be processed in order to perform the processing with high anisotropy in the etching processing. Note that RF is an abbreviation of Radio Frequency and this is referred to as a high frequency ranging from approximately 50 Khz to several tens of Mhz. As in the case of etching process, grooves and holes formed in the substrate surface can be embedded uniformly with a highly dense film by applying the RF bias voltage when forming the thin film. In addition, if the substrate surface is formed with stepped portions, these stepped portions can be eliminated to form a level surface. The reason for this is as follows: When plasma is generated, the so-called floating potential is produced on the substrate surface (or the surface of the thin film formed on the substrate surface) owing to a difference in the mobility of electrons and ions that are present in the plasma, but if RF bias voltage is applied thereto, the magnitude of this floating potential can be controlled, so that it is possible to control the energy of ions directed toward the substrate surface or the thin-film surface. In addition, conceivable as another reason is that when RF bias voltage is applied, the electric fields are produced not only in the vertical direction to the substrate but also in the lateral direction; this effectively acts on the formation of thin film; and the concentration of electric fields facilitates the scraping of pointed portions on the substrate surface.
As the above-described ECR plasma etching/CVD apparatus, one shown in FIG. 9 is known, for example. An outline of the configuration and operation of this apparatus will be described hereinunder. First, a plasma generating chamber 3 and a processing chamber 9 are evacuated by unillustrated evacuating means. N.sub.2 gas is made to flow from gas supplying means 4 into the plasma generating chamber 3, at which time the pulse-like microwaves generated by a microwave generator 17 are introduced via a waveguide 1 serving as transferring means thereof into the plasma generating chamber 3. Provided between the plasma generating chamber 3 and the waveguide 1 is a vacuum window 2 for isolating in an airtight manner the evacuated plasma generating chamber 3 from the waveguide 1 under an atmospheric pressure. The lower portion of the plasma generating chamber 3 is fitted with a metallic plate centrally formed with an opening 7 having a large diameter. The metallic plate and the plasma generating chamber 3 are combined to constitute a half-open microwave resonator. Disposed outside the resonator is an excitation solenoid 6 by which magnetic fields satisfying ECR conditions are produced in the resonator. As a result, the plasma is generated in the resonator. The plasma is forced out into the processing chamber 9 along a transfer path 13 formed by magnetic force. Subsequently, for instance, monosilane gas (SiH.sub.4) is fed from gas supplying means 12 into a space extending to a substrate board 10. Immediately when this gas is activated by the plasma, an active species reacts on a substrate 11 to be processed, to which the RF bias voltage is applied from the RF generator 20, whereby a thin film is formed on the substrate surface. Note that a wire for applying the RF bias voltage to the substrate 11 is covered with a shield of an earth potential, and the peripheral surface of the substrate 11 is surrounded with the shield of the earth potential.
The ECR plasma etching/CVD apparatus is also usable for etching on the substrate by feeding a gas for etching from the gas supplying means 4 instead of N.sub.2 gas.
The following problems are, however, inherent in this type of conventional ECR plasma etching/CVD apparatus. The plasma is generated when the microwaves are introduced into the plasma generating chamber. Hence, when microwave are not generated in between pulse cycles, plasma is not produced. In the case of applying the RF bias voltage, the plasma becomes a load during the occurrence of plasma, and impedance matching is thereby possible. It is therefore possible to apply an adequate voltage onto the substrate. Where no microwave and hence no plasma is generated, however, when seen from the RF generator 20, there is created a no-load state, and the impedance cannot be matched. Meanwhile, if the RF bias voltage is previously adjusted to provide the matching during the occurrence of plasma, the impedance is inevitably mismatched when plasma is not generated, thereby applying a high voltage onto the substrate. This high voltage is in some cases approximately 1 Kv, and electric discharge takes place on the substrate surface. As a result, a first problem arises wherein craters are formed in the substrate surface and breakage results.
In addition, we found that the apparatus also had the following second problem, after having carried out various experiments on plasma processing. While plasma is being generated, the floating potential is produced on the substrate surface, as described above. This floating potential is controlled by the RF bias voltage, and is also dependent on the impedance of the plasma (determined substantially by the electric power of the microwaves supplied) when the RF bias voltage is being applied. Accordingly, when conducting the surface treatment of the substrate by means of ions in the plasma under the action of the electric power of certain microwaves, if an attempt is made to obtain an optimum value of floating potential (i.e., a mean value in time: Since it is difficult for ions in the plasma to follow the RF that is high frequency wave, the mean value in time of the floating potential becomes an issue when the RF bias voltage is being applied), the value (peak value) of the RF bias voltage is unconditionally set at a certain value. Meanwhile, in various types of processing, there are cases where the optimum value (mean value) of the floating potential controlled by this RF bias voltage does not agree with the optimum value (peak value) of the RF bias voltage itself for bringing about that value. That is, if the peak value of the RF bias voltage is set to be a value which produces the floating potential as an optimum mean value, there are cases where the peak value itself of the RF bias voltage becomes an inadequate value. As a result, conditions of processing at the time of effecting the etching or CVD process become difficult to simultaneously satisfy desired processing characteristics of various types, so that there has been a problem in that the results of processing are difficult to control. For instance, in the formation of a thin film, there are cases where even if the peak value of the RF bias voltage is an optimum value with respect to the film quality, stress applied to the film becomes excessively large. In addition, similarly in the etching process as well, there are cases where even if the peak value of the RF bias voltage is an optimum one for improving anisotropy, damage to the substrate surface becomes large due to the sputtering of ions in the plasma.
Accordingly, as an ECR plasma etching/CVD apparatus for overcoming the first problem as described above, an apparatus invented by the same inventors of the present invention and patented earlier in the United States (U.S. Pat. No. 4,891,118, issues Jan. 2, 1990) is shown in FIG. 10. An outline of the arrangement and operation of this apparatus will be described hereinunder.
Those members that are identical with those of FIG. 9 are denoted by the same reference numerals, and a description thereof will be omitted.
In the apparatus shown in FIG. 10, the application of RF bias voltage to a substrate 11 is effected by a synchronization pulse generating circuit 22 in synchronism with the pulses of microwaves. During a time interval when the microwaves are not being produced during pulse cycles of the microwaves, the RF bias voltage is not applied to the substrate.
Hence, in accordance with this apparatus, it is possible to overcome the first problem of the apparatus shown in FIG. 9, i.e., the problem that a high voltage is continuously applied to the substrate, discharge takes place on the substrate surface, and craters are produced in the substrate surface, resulting in breakage.
However, the second problem of the apparatus shown in FIG. 9 cannot be solved by the ECR plasma etching/CVD apparatus shown in FIG. 10. A more detailed explanation will be given of this second problem which was found by the inventors.
As described above, it is only when the microwaves have been introduced into the plasma generating chamber that the plasma is generated. Hence, the RF bias voltage is applied only when the plasma is being generated in the pulse cycles of the microwaves, whereby impedance matching is made with the plasma acting as load, and an adequate voltage is applied to the substrate. With the apparatus shown in FIG. 10, however, the time width for the application of the RF bias voltage at this time is identical with the time width of the microwave pulses, and its magnitude is fixed. If, for instance, an attempt is made to increase the RF bias voltage to be applied to the substrate so as to obtain desired film quality, a high voltage must be output from the RF generator for a time width identical with that of microwave pulses. However, as described above, there are cases where the optimum value as a mean value of the floating potential controlled by the RF bias voltage does not agree with the optimum peak value of the RF bias voltage itself for producing that value. For that reason, in the arrangement as shown in FIG. 10, in which the RF bias voltage of a fixed magnitude is merely generated for the same time width as that of the microwave pulses, although it may be optimum for producing predetermined floating potential, the RF bias voltage itself having a peak value of the magnitude greater than that optimum value must be applied for a long time more than is necessary. As a result, it is impossible to simultaneously satisfy the desired processing characteristics of various types. A specific example of it will be described below.
When forming a thin film on the substrate, for instance, in order to improve the step coverage in the stepped portions of the substrate surface, it is necessary to increase the RF bias voltage to be applied, set the magnitude of floating potential produced on the substrate above a certain threshold value, and deposit a thin film on side walls of the stepped portions by means of the sputtering effect using ions. However, the value of the floating voltage controlled by the RF bias voltage and accelerating the sputtering phenomenon with respect to ions is a mean value in time, as mentioned before. In contrast, the potential which actually appears on the substrate has a relatively large amplitude (peak). Consequently, if an attempt is made to set the peak value of the RF bias voltage applied with a fixed magnitude for the same time width as that of microwave pulses in such a manner as to be a large value so as to create a sufficient value (mean value) of floating potential for producing the sputtering effect, inside the so called sheath which is present between the plasma and the substrate surface, the acceleration and deceleration of the electrons and ions are repeated throughout that time width, and the film quality and the magnitude of stress applied to the film change. As a result, if the RF bias voltage with a large amplitude is applied for a long time more than is necessary, there occurs a case where the film quality and the stress applied to the film do not become adequate. In particular, when forming a thin film of silicon nitride, the stress in the film is liable to be excessively large.
A first object of the present invention is to provide a plasma processing apparatus in which a mean value of floating potential occurring on a substrate surface to be processed can be controlled to a desired value in a wide range, the composition of film, film quality, and stress applied to the film can be arbitrarily controlled in the formation of thin film, and anisotropy, damage to the film, and processing speed can be arbitrarily controlled in etching.
A second object of the present invention is to provide a plasma processing method which is capable of controlling the aforementioned various characteristics in thin film formation or etching, and more particularly to provide a plasma processing method suitable for making adequate the composition of film, film quality, and stress applied to the film in thin film formation and for rendering favorable step coverage realizable.